5.7.3 Callbacks

All choices can have callbacks associated with them. These callbacks are activated when the application user makes a selection, and different sorts of gesture can have different sorts of callback associated with them.

The following callbacks are available:
:selection-callback,
:retract-callback (called when a deselection is made),
:extend-callback, and
:action-callback (called when a double-click occurs). What makes one choice different from another is that they permit different combinations of these callbacks. This is a consequence of the differing interactions. For example, you cannot have an
:extend-callback
in a radio button panel, because you cannot extend selection in one.

Callbacks pass data to the function they call. There are default arguments for each type of callback. Using the
:callback-type keyword allows you to change these defaults. Example values of
callback-type
are
:interface
(which causes the interface to be passed as an argument to the callback function),
:data
(the value of the selected data is passed),
:element
(the element containing the callback is passed) and
:none (no arguments are passed). Also there is a variety of composite
:callback-type
values, such as
:data-interface (which causes two arguments, the data and the interface, to be passed). See the
callbacks
entry in the
LispWorks CAPI Reference Manual
for a complete description of
:callback-type
values.

The following example uses a push button and a callback function to display the arguments it receives.

(defun show-callback-args (arg1 arg2)

(display-message "The arguments were ~S and ~S" arg1 arg2))

(setq example-button

(make-instance 'push-button

:text "Push Me"

:callback 'show-callback-args

:data "Here is some data"

:callback-type :data-interface))

(contain example-button)

Try changing the
:callback-type
to other values.

If you do not use the
:callback-type
argument and you do not know what the default is, you can define your callback function with lambda list
(&rest args)
to account for all the arguments that might be passed.

Specifying a callback that is invalid for a particular choice causes a compile-time error.